9,275 research outputs found
Cosmology and Astrophysics from Relaxed Galaxy Clusters I: Sample Selection
This is the first in a series of papers studying the astrophysics and
cosmology of massive, dynamically relaxed galaxy clusters. Here we present a
new, automated method for identifying relaxed clusters based on their
morphologies in X-ray imaging data. While broadly similar to others in the
literature, the morphological quantities that we measure are specifically
designed to provide a fair basis for comparison across a range of data quality
and cluster redshifts, to be robust against missing data due to point-source
masks and gaps between detectors, and to avoid strong assumptions about the
cosmological background and cluster masses. Based on three morphological
indicators - Symmetry, Peakiness and Alignment - we develop the SPA criterion
for relaxation. This analysis was applied to a large sample of cluster
observations from the Chandra and ROSAT archives. Of the 361 clusters which
received the SPA treatment, 57 (16 per cent) were subsequently found to be
relaxed according to our criterion. We compare our measurements to similar
estimators in the literature, as well as projected ellipticity and other image
measures, and comment on trends in the relaxed cluster fraction with redshift,
temperature, and survey selection method. Code implementing our morphological
analysis will be made available on the web.Comment: MNRAS, in press. 43 pages in total, of which 17 are tables (please
think twice before printing). 18 figures, 4 tables. Machine-readable tables
will be available from the journal and at the url below; code will be posted
at http://www.slac.stanford.edu/~amantz/work/morph14
The Metallicity of the Intracluster Medium Over Cosmic Time: Further Evidence for Early Enrichment
We use Chandra X-ray data to measure the metallicity of the intracluster
medium (ICM) in 245 massive galaxy clusters selected from X-ray and
Sunyaev-Zel'dovich (SZ) effect surveys, spanning redshifts .
Metallicities were measured in three different radial ranges, spanning cluster
cores through their outskirts. We explore trends in these measurements as a
function of cluster redshift, temperature, and surface brightness "peakiness"
(a proxy for gas cooling efficiency in cluster centers). The data at large
radii (0.5--1 ) are consistent with a constant metallicity, while at
intermediate radii (0.1-0.5 ) we see a late-time increase in
enrichment, consistent with the expected production and mixing of metals in
cluster cores. In cluster centers, there are strong trends of metallicity with
temperature and peakiness, reflecting enhanced metal production in the
lowest-entropy gas. Within the cool-core/sharply peaked cluster population,
there is a large intrinsic scatter in central metallicity and no overall
evolution, indicating significant astrophysical variations in the efficiency of
enrichment. The central metallicity in clusters with flat surface brightness
profiles is lower, with a smaller intrinsic scatter, but increases towards
lower redshifts. Our results are consistent with other recent measurements of
ICM metallicity as a function of redshift. They reinforce the picture implied
by observations of uniform metal distributions in the outskirts of nearby
clusters, in which most of the enrichment of the ICM takes place before cluster
formation, with significant later enrichment taking place only in cluster
centers, as the stellar populations of the central galaxies evolve.Comment: 13 pages. Accepted version, to appear in MNRA
Accurate Transfer Maps for Realistic Beamline Elements: Part I, Straight Elements
The behavior of orbits in charged-particle beam transport systems, including
both linear and circular accelerators as well as final focus sections and
spectrometers, can depend sensitively on nonlinear fringe-field and
high-order-multipole effects in the various beam-line elements. The inclusion
of these effects requires a detailed and realistic model of the interior and
fringe fields, including their high spatial derivatives. A collection of
surface fitting methods has been developed for extracting this information
accurately from 3-dimensional field data on a grid, as provided by various
3-dimensional finite-element field codes. Based on these realistic field
models, Lie or other methods may be used to compute accurate design orbits and
accurate transfer maps about these orbits. Part I of this work presents a
treatment of straight-axis magnetic elements, while Part II will treat bending
dipoles with large sagitta. An exactly-soluble but numerically challenging
model field is used to provide a rigorous collection of performance benchmarks.Comment: Accepted to PRST-AB. Changes: minor figure modifications, reference
added, typos corrected
Gilbert Damping in Magnetic Multilayers
We study the enhancement of the ferromagnetic relaxation rate in thin films
due to the adjacent normal metal layers. Using linear response theory, we
derive the dissipative torque produced by the s-d exchange interaction at the
ferromagnet-normal metal interface. For a slow precession, the enhancement of
Gilbert damping constant is proportional to the square of the s-d exchange
constant times the zero-frequency limit of the frequency derivative of the
local dynamic spin susceptibility of the normal metal at the interface.
Electron-electron interactions increase the relaxation rate by the Stoner
factor squared. We attribute the large anisotropic enhancements of the
relaxation rate observed recently in multilayers containing palladium to this
mechanism. For free electrons, the present theory compares favorably with
recent spin-pumping result of Tserkovnyak et al. [Phys. Rev. Lett.
\textbf{88},117601 (2002)].Comment: 1 figure, 5page
Reconstruction of modified gravity with ghost dark energy models
In this work, we reconstruct the modified gravity for different ghost
and generalized ghost dark energy models in FRW flat universe, which describe
the accelerated expansion of the universe. The equation of state of
reconstructed - gravity has been calculated. We show that the
corresponding gravity of ghost dark energy model can behave like phantom
or quintessence. We also show that the equation of state of reconstructed
gravity for generalized ghost model can transit from quintessence regime
to the phantom regime as indicated by recent observations.Comment: 13 pages, some references and one author are added. Accepted for
publication by MPL
First observation of medium-spin excitations in the 138Cs nucleus
Medium-spin, yrast excitations in the 138Cs nucleus, populated in the spontaneous fission of 248Cm, were observed for the first time. 138Cs was studied by means of prompt Îł-ray spectroscopy using the EUROGAM2 array. The newly observed yrast cascade, built on the known 6- isomer at 80 keV, was successfully described by shell model calculations. Analogously to the 136I isotone, the 6- isomer in 138Cs has the \ensuremath{(\pi g_{7/2} ^4 d_{5/2} \nu f_{7/2})_{6^-}} dominating configuration and the 7- excitation, located 175 keV above, corresponds to the \ensuremath{(\pi g_{7/2} ^3 d_{5/2}^2 \nu f_{7/2})_{7^-}} as dominating configuration. Similarly as in 136I, changing the position of the d
5/2 proton orbital improves the reproduction of the data. However, in 138Cs the energy of this orbital should be increased compared to its energy in 133Sb, to get the best description, in contrast to 136I and 135Sb, where it had to be decreased. The best reproduction of excitation energies in 138Cs is obtained assuming that the Ïd
5/2 orbital in 138Cs is located about 100 keV higher than in 133Sb. These observations suggest that the lowering of the d
5/2 s.p. energy in 135Sb is not a physical effect due to the appearance of a neutron skin, as proposed by other authors, but rather an artifact due to some deficiency of the input data used in the shell model calculations in the region of the doubly magic 132Sn core
Design Principles for Aqueous Interactive Materials: Lessons from Small Molecules and Stimuli-Responsive Systems.
Interactive materials are at the forefront of current materials research with few examples in the literature. Researchers are inspired by nature to develop materials that can modulate and adapt their behavior in accordance with their surroundings. Stimuli-responsive systems have been developed over the past decades which, although often described as "smart," lack the ability to act autonomously. Nevertheless, these systems attract attention on account of the resultant materials' ability to change their properties in a predicable manner. These materials find application in a plethora of areas including drug delivery, artificial muscles, etc. Stimuli-responsive materials are serving as the precursors for next-generation interactive materials. Interest in these systems has resulted in a library of well-developed chemical motifs; however, there is a fundamental gap between stimuli-responsive and interactive materials. In this perspective, current state-of-the-art stimuli-responsive materials are outlined with a specific emphasis on aqueous macroscopic interactive materials. Compartmentalization, critical for achieving interactivity, relies on hydrophobic, hydrophilic, supramolecular, and ionic interactions, which are commonly present in aqueous systems and enable complex self-assembly processes. Relevant examples of aqueous interactive materials that do exist are given, and design principles to realize the next generation of materials with embedded autonomous function are suggested.JAM thanks ESPRC for an IAA KTF
M is grateful for a Newton International Fellowship
OAS is thankful to ERC Consolidator Grant CAM-RI
Early Advanced LIGO binary neutron-star sky localization and parameter estimation
2015 will see the first observations of Advanced LIGO and the start of the
gravitational-wave (GW) advanced-detector era. One of the most promising
sources for ground-based GW detectors are binary neutron-star (BNS)
coalescences. In order to use any detections for astrophysics, we must
understand the capabilities of our parameter-estimation analysis. By simulating
the GWs from an astrophysically motivated population of BNSs, we examine the
accuracy of parameter inferences in the early advanced-detector era. We find
that sky location, which is important for electromagnetic follow-up, can be
determined rapidly (~5 s), but that sky areas may be hundreds of square
degrees. The degeneracy between component mass and spin means there is
significant uncertainty for measurements of the individual masses and spins;
however, the chirp mass is well measured (typically better than 0.1%).Comment: 4 pages, 2 figures. Published in the proceedings of Amaldi 1
Habitat corridors facilitate genetic resilience irrespective of species dispersal abilities or population sizes
Corridors are frequently proposed to connect patches of habitat that have become isolated due to humanâmediated alterations to the landscape. While it is understood that corridors can facilitate dispersal between patches, it remains unknown whether corridors can mitigate the negative genetic effects for entire communities modified by habitat fragmentation. These negative genetic effects, which include reduced genetic diversity, limit the potential for populations to respond to selective agents such as disease epidemics and global climate change. We provide clear evidence from a forwardâtime, agentâbased model (ABM) that corridors can facilitate genetic resilience in fragmented habitats across a broad range of species dispersal abilities and population sizes. Our results demonstrate that even modest increases in corridor width decreased the genetic differentiation between patches and increased the genetic diversity and effective population size within patches. Furthermore, we document a tradeâoff between corridor quality and corridor design whereby populations connected by highâquality habitat (i.e., low corridor mortality) are more resilient to suboptimal corridor design (e.g., long and narrow corridors). The ABM also revealed that species interactions can play a greater role than corridor design in shaping the genetic responses of populations to corridors. These results demonstrate how corridors can provide longâterm conservation benefits that extend beyond targeted taxa and scale up to entire communities irrespective of species dispersal abilities or population sizes.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/111750/1/eva12255.pd
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